Belfast , Northern Ireland -LRB- CNN -RRB- -- Protests broke out Friday throughout Northern Ireland against a decision by Belfast City Council members to stop flying the Union flag year-round .

`` Most of the protests were peaceful , although disruption was caused to the road network in Belfast and across the country , '' police said in a statement .

One of the most serious incidents occurred at O'Neil 's Road in Newtownabbey , where about 100 Protestant rioters -- many of them teenagers -- threw concrete blocks , paving slabs , bricks , planks of wood , gasoline bombs and Molotov cocktails at police .

They targeted about 30 armored police vehicles and as many as 100 police officers clad in riot gear . At least one officer appeared injured . His head bowed , the officer was helped away by comrades .

Amid rain and temperatures just above freezing , police brought in two water cannon , deploying one of them to push back the rioters . Police were trying to gain control of a main highway .

In pictures : Flag protests in Northern Ireland

In all , four police officers were hurt , with one requiring hospital treatment , and two people were arrested , police said . Thirty-three gasoline bombs were thrown at police during the demonstrations , after which all the roads were reopened .

Read more : Clinton urges calm amid tensions in Northern Ireland

The eruptions occurred after two nights of calm that followed five consecutive nights of rioting .

Belfast has been at the center of a wave of protests prompted by the City Council 's December 3 decision to limit the flying of the flag to 18 days per year .

The disorder has resulted in more than 100 arrests and left 66 officers injured .

The flag was raised Wednesday for the first time since the controversial vote to mark the birthday of Prince William 's wife , Catherine . It was lowered at the end of the day .

The leaders of the two main unionist political parties met Thursday to discuss concerns within the unionist community .

Read more : Political forum aims to stem N.Ireland violence

Northern Ireland authorities have accused loyalist extremists of exploiting the decision by Belfast officials to end the century-old tradition of flying the Union flag over City Hall 365 days a year .

The British flag has long been a flashpoint between British loyalists and Irish nationalists , who want Northern Ireland to join the Irish republic .

The Belfast City Council vote followed a summer of heightened tensions between Northern Ireland 's Catholic and Protestant communities . Riots in September left dozens of police injured .

The majority of Ireland gained independence in 1921 following two years of conflict . But six of the nine counties of the province of Ulster chose to stay in the United Kingdom , eventually becoming Northern Ireland .

Police : Extremist group ` orchestrating violence ' in Belfast over Union Jack

In the late 1960s , the conflict between mainly Protestant loyalists , who want Northern Ireland to remain part of the United Kingdom , and largely Roman Catholic nationalists , who want it to be reunited with the rest of Ireland , exploded into a political and sectarian war , known as `` the Troubles . ''

The three decades of ensuing violence between loyalists and the IRA claimed more than 3,000 lives , most of them north of the border . While the Good Friday Agreement of 1998 , also known as the Belfast Agreement , effectively ended the conflict , distrust remains between Catholics and Protestants .

Under the terms of the accord , groups on both sides dumped their weapons , and members of Sinn Fein , the political affiliate of the IRA , now work with pro-British politicians in Northern Ireland 's power-sharing government .

CNN 's Laura Smith-Spark wrote in London and Nic Robertson and Peter Taggart reported in Belfast .

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NEW : Four police officers are hurt , one is hospitalized

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NEW : Two people are arrested

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Protesters angry over a decision to stop flying the Union flag year-round

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Police say 66 officers injured , 100 arrests made since December